1. Technical Field
The present disclosure relates to an antenna device.
2. Description of the Related Art
Conventionally, on-vehicle radar apparatuses have been known as radar apparatuses that detect obstacles by transmitting and receiving electromagnetic waves. An on-vehicle radar apparatus is used, for example, for detecting an obstacle while the vehicle is moving and for detecting a vehicle that is passing from behind. In a case where such a radar apparatus is attached to a vehicle, it is often installed in a bumper of the vehicle. However, in a case where the radar apparatus is installed in the bumper of the vehicle, the radar apparatus deteriorates in detection performance, as the bumper reflects an electromagnetic wave.
As a method for preventing the radar apparatus from deteriorating in detection performance due to a cover member of the bumper, Japanese Unexamined Patent Application Publication No. 2003-240838, for example, discloses a technology for optimizing the thickness of the cover member.
However, even with the optimization of the thickness of the cover member, the radar apparatus deteriorates in detection performance, for example, due to the difference in the reflectance of an electromagnetic wave attributed to the fact that coating agents that are applied to the outer sides of cover members vary from one type of vehicle to another and due to the difference in the reflectance of an electromagnetic wave attributed to the fact that individual cover members vary in thickness.
Further, an electromagnetic wave reflected by the cover member undergoes multiple reflections between the cover member and an antenna device of the radar apparatus. The state of the multiple reflections depends on the distance between the cover member and the antenna device.
The distance between the cover member and the antenna device changes according to variations in installation operation at the time of installation of the radar apparatus. Furthermore, in the case of a vehicle, the distance between the cover member and the antenna device also changes according to vibrations while the vehicle is moving. For this reason, changes in the distance between the cover member and the antenna device as caused by these factors cause variations in detection performance of the radar apparatus.